Internal redox property

This considerable enhancement in redox properties may however remain chemically hidden. Several causes may converge to mask these properties. First of all electron transfer is an intermolecular act of reactivity even when thermodynamically feasible it may have to compete with very rapid intramolecular acts of deactivation (fluorescence, phosphorescence, internal conversion)99. The rate of electron transfer is given by the Rehm-Weller equation96,100... 

The model shown in Scheme 2 indicates that a change in the formal oxidation state of the metal is not necessarily required during the catalytic reaction. This raises a fundamental question. Does the metal ion have to possess specific redox properties in order to be an efficient catalyst A definite answer to this question cannot be given. Nevertheless, catalytic autoxidation reactions have been reported almost exclusively with metal ions which are susceptible to redox reactions under ambient conditions. This is a strong indication that intramolecular electron transfer occurs within the MS"+ and/or MS-O2 precursor complexes. Partial oxidation or reduction of the metal center obviously alters the electronic structure of the substrate and/or dioxygen. In a few cases, direct spectroscopic or other evidence was reported to prove such an internal charge transfer process. This electronic distortion is most likely necessary to activate the substrate and/or dioxygen before the actual electron transfer takes place. For a few systems where deviations from this pattern were found, the presence of trace amounts of catalytically active impurities are suspected to be the cause. In other words, the catalytic effect is due to the impurity and not to the bulk metal ion in these cases. 

As mentioned above, treatment of the aldol adducts 150 a/b with NMO produced the phenol 152. The interesting oxidation properties of NMO had previously been investigated by Sulikowski et al. on the model compound 157 [85] (Scheme 40). They observed the formation of the hemiacetal 159 in 60% yield and assumed attack of the nucleophilic N-oxide on the quinonemethide tautomer 158 (or on the anion of 158). A related reaction was observed in our group in which the diol 94 was methoxylated at C-6 to 95 by treatment with methoxide ions [82] (Scheme 27). An internal redox step is postulated to account for the reductive 0-N-bond cleavage with concomitant oxidation of the hydroquinone back to the quinone. Without the presence of perruthenate, aromatization with formation of a C-5 phenolic hydroxy group was observed, a reaction later exploited in the synthesis of angucycline 104-2 [87] (see Scheme 49). Thus, based on similar mechanistic principles, the chemical results of the NMO oxidations were quite different compound 147 gave the C-6 phenol 152 [86] whereas 157/158 were converted to the C-5 phenol 160 [85]. 

In summary, the comparatively small internal reorganization energy of C o, together with its excellent redox properties, should make the Cgo chromophore a valuable electron acceptor component in molecular photovoltaic devices. 

Peeters MPJ, van Hooff JHC, Sheldon RA, Zholobenko VL, Kustov LM, Kazansky VB (1993) Spectroscopic investigations of the redox properties of CoAPO molecular sieves. In von Ballmoos R, Higgins JB, Treacy MMJ (eds) Proceedings from the Ninth International Zeolite Conference. Butterworth-Heinemann, Boston London Oxford Singapore Sydney Toronto Wellington, vol I, p 651... 

Grubert G, Griinert W, Rathousky J, Zukal A, Schulz-Ekloff G, Wark M (1999) Structme and redox properties of vanadimn species in MCM-41. In Treacy MMJ, Marcus BK, Bisher ME, Higgins JB (eds) Proceedings of the Twelfth International Zeolite Conference. Materials Research Society, Warrendale, vol II, p 825... 

It has been already emphasized that substitution of heteroelements into the framework of molecular sieves creates acidic sites. Incorporation of transition elements such as Ti, V, Mn, Fe, or Co, which have redox properties, provides molecular sieves with redox active sites that are involved in oxidation reactions (323-332). As mentioned in the beginning of the article, the transition metal-substituted molecular sieves, the so-called redox molecular sieves, exhibit several advantages compared with other types of heterogeneous redox catalysts (1) redox sites are isolated in a well-defined internal structure therefore, oligomerization of the active oxometal species is prevented (this is a major reason for the deactivation of homogeneous catalysts) (2) the site isolation (the so-called microenvironment) of redox centers prevents the leaching of the metal ions, which frequently happens in liquid-phase oxidations catalyzed by conventional transition metal-supported catalysts (3) well-defined cavities and channels of molecular dimensions endow the catalysts with unique performances such as the shape selectivity (and traffic control) toward reactants, intermediates, and/or products. 

Catlow, C. R. A. The effects of dopants on the redox properties of oxide fuel. Proc. IAEA Specialists Meeting on Internal I el Rod Chemistry, Erlangen, FRG, 1979 IAEA Report IWG FPT/3, p. 127-130 (1979)... 

RhCl(TPPMS Na )3 is an orange-yellow crystalline material, highly soluble in water, practically insoluble in anhydrous THF, ethanol, acetone or diethyl ether. As a soUd, it is moderately stable to air oxidation and can be kept for several months in a dry, cool place with no decomposition. In aqueous solutions, RhCl(TPPMS)3 is rapidly oxidized by O2. Moreover, it undergoes internal redox processes, even in anaerobic conditions, yielding Rh(III)-con-taining products and phosphine oxide. Such decomposition is accelerated in basic solutions and by heating. Spectral properties UV (0.1 M HCl) 356 nm (e 3.2x 10 M cm ), 444 nm (e 8.0x 10 cm ) IR... 

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